It is known that hydrogen peroxide may be prepared by direct synthesis from gaseous mixtures containing hydrogen and oxygen by reacting the gaseous mixture on a noble metal catalyst in the presence of a liquid aqueous, aqueous-organic or organic reaction medium. The organic or aqueous-organic hydrogen peroxide solutions obtainable by the direct synthesis process are of interest as oxidizing agents for the catalytic oxidation of organic compounds.
One problem with the direct synthesis of hydrogen peroxide is that the noble metal catalysts used have an unwanted catalytic activity in decomposing hydrogen peroxide to water and oxygen. This activity can be inhibited by adding a sufficient concentration of a strong acid and halide to the liquid reaction medium. However, the addition of acid and halide causes the reaction medium to become strongly corrosive towards metallic materials, particularly stainless steel. One problem that this creates is that, although metal reactors are desirable for use in processes on an industrial scale because of their ability to dissipate heat, corrosion of the reactor material gives rise to safety problems. This is especially true if the direct synthesis is carried out at elevated pressure, which is desirable for achieving a high space-time yield. In addition, metal ions dissolved out of the reactor material by corrosion have an adverse effect on the stability of the hydrogen peroxide solutions prepared, and may cause unwanted secondary reactions when the solutions are used in oxidation reactions.
The problem of corrosiveness due to the simultaneous presence of acid and halide ions in the reaction medium is known from EP-A 0 978 316 and EP-A 1 344 747, both of which suggest carrying out the reaction in the presence of special catalysts in which the noble metal is applied to a catalyst support containing acid groups. Although the use of the proposed catalyst supports makes it possible to lower the acid concentration in the reaction medium, it is still necessary to add 3 to 34 ppm of HBr in order to effectively inhibit the decomposition activity of the catalyst, and therefore the liquid reaction mixture remains corrosive. Because of this corrosiveness, EP-A 0 978 316 suggests using autoclaves which are either made of the particularly corrosion-resistant alloy Hastelloy C or are made of stainless steel and have a glass insert to contain the liquid reaction medium. The use of these materials or inert reactor coatings, for example glass or PTFE, increases production costs and may make the process uneconomical. There is therefore a need for a process for the direct synthesis of hydrogen peroxide in the presence of acid and halide that can be carried out in a reactor made of a commercially available stainless steel material without corrosion.
It is known from DE-A 196 42 770 to carry out direct synthesis reactions in a methanolic reaction medium containing 0.4 wt. % of sulfuric acid, 0.1 wt. % of phosphoric acid and 6 ppm of bromide in the form of sodium bromide, and to use a shaped body made of V4A steel fabric coated with palladium metal as the catalyst. Because of the coating, the V4A steel fabric used as the catalyst support is not in direct contact with the corrosive reaction mixture. The document does not teach how to avoid corrosion of stainless steel on direct contact with the corrosive reaction mixture.
DE-A 198 57 137 describes the direct synthesis of hydrogen peroxide in a methanolic reaction medium containing 0.3 wt. % of sulfuric acid, 0.03 wt. % of phosphoric acid and 5 ppm of bromide in the form of sodium bromide. The reaction is carried out in a V4A steel autoclave operated as a continuous stirred tank. The autoclave is operated in this case with a gas stirrer to disperse the gaseous mixture in the liquid reaction medium. For a construction of this type, the autoclave has to contain a gas cushion in the region of the cover so that the gas stirrer can suck in gas and distribute it in the reaction mixture. '137 does not disclose the period of time over which the direct synthesis was carried out in the V4A stirred autoclave. The document also does not disclose the extent to which corrosion occurred on the reaction vessel or how to avoid corrosion.
WO 03/082458 describes a process for the direct synthesis of hydrogen peroxide in which a gaseous mixture and a liquid reaction medium are passed at high velocity through a tubular reactor so that a coherent gas phase does not form in the reactor. As materials suitable for the reactor, the document mentions duplex stainless steels having a PREN number of more than 34. Since duplex stainless steels have a substantially higher chromium and molybdenum content than commercially available austenitic stainless steels, they are more expensive and also less readily available. Because of the necessary high flow velocity of 1.2 m/s or more, the process described in WO 03/082458 also requires particularly long and hence expensive reactors. Thus, for example, a pressure reactor with a length of more than 2000 m is used in Example 1.